Adult S/M/L, but you could easily convert the pattern for wee feet by switching to worsted yarn and a 7 needle. Make your thrums a little thinner (max of 2 wraps) and you should be set.

To scale to kiddie sizes, use thinner yarn & needles. With Valley Yarns Berkshire (heavy worsted) and US 8 at 17 st over 4” in plain stockinette (no thrums), a size small produced an 8” slipper (for a 6.5 - 9” foot). The picture with the peacock blue slipper shows that example in size small side-by-side with the original adult small.

You can scale down further with lighter yarns, or skip the thrums. You knit the sole first, which has about the same stitches as your average swatch, and that will immediately let you know if you’re on the right track for size.

If you experiment with gauge, let me know what you find & I’ll post the results for everyone!